Noise estimation in communication receivers

ABSTRACT

A method includes receiving at a receiver a signal including reference symbols that is sent over a communication channel from a transmitter to the receiver. A response of the communication channel is estimated by applying one or more weighting values to the reference symbols. A noise correction factor is computed based on the weighting values. An estimate of a noise level in the received signal is computed based on the estimated response of the communication channel and the noise correction factor. The received signal is decoded based on the estimate of the noise level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/831,282, filed Jul. 7, 2010, now U.S. Pat. No. 8,526,552, whichclaims the benefit of U.S. Provisional Patent Application 61/236,823,filed Aug. 25, 2009. The disclosures of these related applications areincorporated herein by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to communication systems, andparticularly to methods and systems for noise estimation incommunication receivers.

BACKGROUND

Communication receivers commonly estimate the response of thecommunication channel over which they receive signals, as well as thenoise level in the received signals. Channel and noise estimation areused, for example, for decoding the received signals and for assessingthe channel quality.

SUMMARY

An embodiment that is described herein provides a method, which includesreceiving at a receiver a signal including reference symbols that issent over a communication channel from a transmitter to the receiver. Aresponse of the communication channel is estimated by applying one ormore weighting values to the reference symbols. A noise correctionfactor is computed based on the weighting values. An estimate of a noiselevel in the received signal is computed based on the estimated responseof the communication channel and the noise correction factor. Thereceived signal is decoded based on the estimate of the noise level.

In some embodiments, the method includes calculating channel qualityfeedback for the communication channel based on the estimated responseof the communication channel and the estimate of the noise level. In anembodiment, receiving the signal includes receiving two or more versionsof the signal via two or more antennas, respectively, computing theestimate of the noise level includes calculating multiple noisecovariances based on the versions of the signal, and computing the noisecorrection factor includes calculating multiple noise correction factorsto be applied respectively to the noise covariances.

In a disclosed embodiment, applying the weighting values includesselecting the weighting values from among multiple sets of the weightingvalues. In an embodiment, computing the estimate of the noise levelincludes applying a single noise correction factor for at least two ofthe sets of the weighting values. In another embodiment, the methodincludes choosing one or more of the weighting values so as to improvethe estimate of the noise level. In yet another embodiment, choosing theweighting values includes defining a maximum value for the noisecorrection factor, and adjusting the weighting values such that thenoise correction factor does not exceed the maximum value.

In still another embodiment, computing the noise correction factorincludes selecting the noise correction factor from a list of predefinednoise correction factors by applying a selection criterion to theweighting values. In an embodiment, receiving the signal includesreceiving a Long Term Evolution (LTE) signal.

There is additionally provided, in accordance with an embodiment that isdescribed herein, apparatus including a receiver front-end, a channelestimation unit, a correction unit and a noise estimation unit. Thereceiver front-end is configured to receive a signal including referencesymbols that is sent from a transmitter to the receiver over acommunication channel. The channel estimation unit is configured toestimate a response of the communication channel by applying one or moreweighting values to the reference symbols. The correction unit isconfigured to compute a noise correction factor based on the weightingvalues. The noise estimation unit is configured to compute an estimateof a noise level in the received signal based on the estimated responseof the communication channel and the noise correction factor. In anembodiment, a mobile communication terminal includes the disclosedcommunication apparatus. In another embodiment, a chipset for processingsignals in a mobile communication terminal includes the disclosedcommunication apparatus.

The present disclosure will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that schematically illustrates a communicationsystem, in accordance with an embodiment that is described herein; and

FIG. 2 is a flow chart that schematically illustrates a method forsignal reception using noise estimate correction, in accordance with anembodiment that is described herein.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments that are described herein provide improved methods anddevices for estimating noise in communication receivers. In someembodiments, a mobile communication terminal receives a downlink signalover a communication channel. The received signal comprisesdata-carrying symbols and reference symbols. The terminal estimates theresponse of the communication channel and the noise level in the signalbased on the received reference signals. The noise and channel responseestimates are used for decoding the data-carrying symbols and forassessing the channel quality.

In many practical scenarios, however, estimating both the noise and thechannel response using the same reference symbols distorts the noiseestimate. In some embodiments, the terminal corrects this distortion bycalculating a noise correction factor and applying this factor to thenoise estimate.

In an example embodiment, the channel response is estimated by assigningrespective weights to the received reference symbols, and calculatingthe estimated channel response based on the weighted reference symbols.Assigning different weights to different reference symbols enables theterminal to achieve accurate channel estimation for variouscommunication channels and various channel conditions, e.g., delayspread, Signal-to-Noise Ratio (SNR) and speed of the communicationterminal. The channel estimation weights may depend on thetime-frequency location of the reference symbols, the rate at which thechannel varies over time, the SNR conditions and/or the channel timeprofile, for example. In this embodiment, the terminal calculates thenoise correction factor as a function of the weights used for channelestimation. Thus, the corrected noise estimate is calculated based onthe received reference symbols, the estimated channel response and thenoise correction factor. In one example embodiment, the noise correctionfactors can be different per frequency location as the channelestimation weights can change over frequency. In another exampleembodiment, the noise correction factors can be similar over frequency.

The techniques described herein improve the accuracy of the noiseestimate, even though the estimate is based on the same referencesymbols that are used for channel estimation. As a result, communicationreceivers that use these techniques are able to decode the data carriedby the received signals with low error probability, and to assess thechannel quality (e.g., for computing channel quality feedback) with highaccuracy.

FIG. 1 is a block diagram that schematically illustrates a communicationsystem 20, in accordance with an embodiment that is described herein.System 20 comprises a mobile communication terminal 24, also referred toas User Equipment (UE). UE 24 receives downlink signals from a BaseTransceiver Station (BTS) 28 over a wireless communication channel. UE24 may comprise, for example, a cellular phone, a communication-enabledmobile computing device, a cellular adapter for a mobile computingdevice, or any other suitable communication terminal.

In the present example, system 20 comprises an Evolved UniversalTerrestrial Radio Access (E-UTRA) system, also referred to as Long-TermEvolution (LTE). In this embodiment, the downlink signals received by UE24 comprise Orthogonal Frequency Division Multiplexing (OFDM) signals.In alternative embodiments, system 20 may conform to any other suitablecommunication standard or protocol, such as Wideband Code-DivisionMultiple Access (WCDMA), WiMAX and LTE-Advanced (LTE-A). The downlinksignals that are transmitted from BTS 28 to UE 24 comprise data-carryingsymbols and reference symbols. The reference symbols are symbols whosevalues and time-frequency locations are known to UE 24 a-priori, and aretypically used by the UE for various estimation processes.

In UE 24, a receiver front-end (RX FE) 36 receives the downlink signalsvia an antenna 32. RX FE 36 typically down-converts, filters anddigitizes the received signals, so as to produce a digital basebandsignal. The baseband signal output by the RX FE comprises bothdata-carrying symbols and reference symbols. The reference symbols aredenoted Y.

UE 24 comprises a channel estimation unit 40 and a noise estimation unit44, both operating on the received reference symbols. Channel estimationunit 40 estimates the response of the wireless communication channelbetween BTS 28 and UE 24 based on the received reference symbols. Theestimated channel response is denoted

. The term “channel response” refers to the gain and transfer phase ofthe channel between the transmitted signal generated at the BTS and thereceived signal at the UE. The gain and phase are sometimes expressed asa complex value. In some embodiments, the BTS and/or UE comprisemultiple antennas. In these embodiments, unit 44 typically estimates thechannel response for each pair of transmit antenna and receive antenna.

In an embodiment, the channel estimation unit applies certain weights(also referred to as weighting values) to the received referencesymbols, and then estimates the channel response based on the weightedreference symbols. The weights are denoted W. In an example embodiment,unit 40 estimates the channel response using the weights and thereceived reference symbols by calculating a weighted average of thereference symbols in which each reference symbol is multiplied by therespective weight. The channel estimation unit may use differentapproaches to estimate the channel response, e.g., one-dimensional (1-D)time-domain and 1-D frequency-domain calculations, or two-dimensional(2-D) calculations. In both cases the channel estimation weights thatare used to correct the noise estimation can be viewed as equivalent 2-Dweights.

Noise estimation unit 44 uses the received reference symbols to estimatethe noise level in the received downlink signals. In context of thepresent patent application and in the claims, the term “noise level”refers to the level of any undesired component of the received signal,e.g., thermal noise, interference from undesired transmissions, noisethat is added to the received signal due to various receiverimperfections (e.g., channel estimation that differs from the actualchannel value and may add noise to the decoded signal), and/or any otherundesired component. In an example embodiment, unit 44 estimates thenoise variance of the received signal at each receive antenna. In someembodiments, the noise estimation is frequency-dependent.

The outputs of units 40 and 44 are used for decoding the data-carryingsymbols of the downlink signals, as well as for assessing the channelquality. In an embodiment, UE 24 comprises an equalizer/demodulator 48,which demodulates the data-carrying symbols based on the estimatedchannel response and the estimated noise level, so as to decode the datatransmitted from BTS 28 to UE 24. UE 24 further comprises a ChannelQuality Indication (CQI) calculation unit 52, which calculates the CQIof the communication channel based on the estimated channel response andthe estimated noise level. In some embodiments, the CQI is fed back toBTS 28.

As noted above, both channel estimation and noise estimation areperformed by operating on the received reference symbols Y. In thepresent example, the OFDM downlink signals, including the referencesymbols, are received in certain time-frequency allocations that arereferred to as Resource Elements (REs). In practice, deriving bothchannel and noise estimates from the same reference symbols may distortthe noise estimation.

Consider, for example, the downlink signal that is received in a certainRE. The received signal can be written asY=S·H+N  Equation 1wherein Y denotes the received signal, S denotes the signal transmittedby the BTS, H denotes the channel response, and N denotes the noiseterm. Assume that unit 44 were to estimate the noise based on thereceived reference symbols Y and the estimated channel response g:{circumflex over (N)}=Y−Ĥ·S.  Equation 2

In some practical cases, this sort of noise estimation deviates from thetrue noise level, because the channel estimate and the noise estimateare derived from the same reference symbols in the same REs. Errors inthe noise estimation are especially likely when the noise estimate isexpected to measure the sum of the noise variance and the channelestimation errors, as is often the case in practice.

Consider, for example, two reference symbols m=1 and m=2, which arereceived in two different REs. In the present example, the channel isconstant, i.e., H(m)=1, and the transmitted reference symbols are alsoconstant, i.e., S(m)=1. According to Equation 1 above, the receivedsignal is given by Y(1)=1+N(1) and Y(2)=1+N(2).

In the present example, channel estimation unit 40 estimates the channelresponse using the following weighted average:

$\begin{matrix}{{\hat{H}(1)} = {{\frac{3}{4} \cdot {Y(1)}} + {\frac{1}{4} \cdot {{Y(2)}.}}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

The noise estimate in this case (using Equation 2 above) is:

$\begin{matrix}{{\hat{N}(1)} = {{{Y(1)} - \left( {{\frac{3}{4} \cdot {Y(1)}} + {\frac{1}{4} \cdot {Y(2)}}} \right)} = {{\frac{1}{4} \cdot {N(1)}} - {\frac{1}{4} \cdot {{N(2)}.}}}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

Assume that there is no correlation between N(1) and N(2), and that bothnoise terms have the same variance. Thus, the estimated noise variancewould be the empirical variance of {circumflex over (N)}(1) thatconverges asymptotically to

$\begin{matrix}{{{\hat{\sigma}}_{N}^{2}(1)} = {\frac{2}{16}{\sigma_{N}^{2}.}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

If, however, the channel estimation errors and the noise term areactually uncorrelated, then the true value of the noise variance shouldbe:

$\begin{matrix}\begin{matrix}{{{\overset{\sim}{\sigma}}_{N}^{2}(1)} = {\sigma_{N}^{2} + {E\left\{ {{H - \hat{H}}}^{2} \right\}}}} \\{= {{\sigma_{N}^{2}\left( {1 + \frac{10}{16}} \right)} = {\frac{26}{16}{\sigma_{N}^{2}.}}}}\end{matrix} & {{Equation}\mspace{14mu} 6}\end{matrix}$

By comparing Equations 5 and 6, it can be seen that there is aconsiderable deviation (a factor of thirteen) between the estimatednoise variance (Equation 5) and the true noise variance (Equation 6). Inan OFDM system, such a deviation may occur since the noise is estimatedusing the same pilot bins that are also used for channel estimation.Thus, the noise estimation may be degraded due to presence of the actualnoise term both in the channel estimation and in the noise estimation.

In some embodiments, UE 24 comprises a noise correction unit 56, whichcalculates a noise correction factor that compensates for theabove-described errors in the noise estimate of unit 44. The noisecorrection factor is denoted B. Noise correction unit 56 accepts theweights W that are applied to the reference symbols by channelestimation unit 40, and calculates the correction factor B based on theweights W.

Noise estimation unit 44 then corrects its noise estimate using thecorrection factor B, so as to produce a corrected noise estimate. Inother words, noise estimation unit 44 calculates the noise estimatebased on (1) the received pilot symbols Y, (2) the estimated channelresponse Ĥ, and (3) the noise correction factor B. The corrected noiseestimate is then used by equalizer/demodulator 48 for decoding the data,and by CQI calculation unit 52 for assessing the channel qualityfeedback.

The UE configuration shown in FIG. 1 is a simplified exampleconfiguration, which is depicted solely for the sake of conceptualclarity. In alternative embodiments, any other suitable UE configurationcan be used. UE elements that are not necessary for understanding thedisclosed techniques have been omitted from the figure for the sake ofclarity. For example, the UE typically comprises transmission elements(not shown in the figure) for transmitting uplink signals toward theBTS. Although FIG. 1 shows a single UE and a single BTS for the sake ofclarity, real-life systems typically comprise multiple UEs and multipleBTSs.

The different elements of UE 24, including units 40, 44, 48, 52 and 56,may be implemented using dedicated hardware logic, such as using one ormore Application-Specific Integrated Circuits (ASICs) and/orField-Programmable Gate Arrays (FPGAs). UE elements can be combined withone another in one or more IC devices, or partitioned among different ICdevices in any suitable manner. Alternatively, some UE elements may beimplemented using software running on general-purpose hardware, or usinga combination of hardware and software elements. In some embodiments,certain elements of UE 24 are implemented using a general-purposeprocessor, which is programmed in software to carry out the functionsdescribed herein, although it too may be implemented on dedicatedhardware. The software may be downloaded to the processor in electronicform, over a network, for example, or it may, alternatively oradditionally, be provided and/or stored on non-transitory tangiblemedia, such as magnetic, optical or electronic memory. In someembodiments, some or all of the elements of UE 24 may be fabricated in achip-set.

The following description demonstrates the process of calculating thenoise correction factor and using it to correct the noise estimate. Inthe present example, the UE receives the downlink signals via multiplereceive antennas, and the noise estimate and noise correction factorsare calculated for each noise covariance element. For example:R _(NN)(i,j)=E{N _(i) ·N _(j)*}  Equation 7wherein R_(NN)(i,j) denotes the noise covariance term between the i^(th)and j^(th) receive antennas. Typically, the noise estimation unitestimates the noise by calculating noise covariance between pairs ofantennas, and the noise correction unit calculates the noise correctionfactor per noise covariance element. In an example embodiment, the noisecorrection factors are calculated from the channel estimation weightingfactors.

In this embodiment, unit 40 estimates the channel response bycalculating a linear weighted average of the received reference symbols.In an example embodiment, the weights are assigned using a MMSEcriterion. The channel response estimate can be written as:

$\begin{matrix}{{{\hat{H}(m)} = {\sum\limits_{k}{{w(k)} \cdot {Y(k)}}}},} & {{Equation}\mspace{14mu} 8}\end{matrix}$wherein Ĥ(m) denotes the channel response estimate for the m^(th) RE,based on two or more reference symbols. The estimated noise (beforecorrection) can be written as:

$\begin{matrix}\begin{matrix}{{\hat{N}(m)} = {{Y(m)} - {\sum\limits_{k}{{w(k)} \cdot \left( {{H(k)} + {N(k)}} \right)}}}} \\{= {{H(m)} - {\sum\limits_{k}{{w(k)} \cdot {H(k)}}} + {N(m)} - {\sum\limits_{k}{{w(k)} \cdot {N(k)}}}}}\end{matrix} & {{Equation}\mspace{14mu} 9} \\\begin{matrix}{{\hat{N}(m)} = {{\Delta\;{H(m)}} + {N(m)} - {\sum\limits_{k}{{w(k)} \cdot {N(k)}}}}} \\{= {{\Delta\;{H(m)}} + {{N(m)} \cdot \left( {1 - {w(m)}} \right)} - {\sum\limits_{k \neq m}{{w(k)} \cdot {N(k)}}}}}\end{matrix} & {{Equation}\mspace{14mu} 10}\end{matrix}$

The output of noise estimate can thus be written as

$\begin{matrix}{{R_{\hat{N}\hat{N}}\left( {{\mathbb{i}},j} \right)} = {{E\left\{ {{\hat{N}}_{i} \cdot {\hat{N}}_{j}^{*}} \right\}} = {{E\left\{ {\Delta\;{H_{i} \cdot \Delta}\; H_{j}^{*}} \right\}} + {E{\left\{ {N_{i} \cdot N_{j}^{*}} \right\} \cdot \left( {1 - w_{i} - w_{j}^{*} + {w_{i} \cdot w_{j}^{*}}} \right)}} + {E{\left\{ {N_{i} \cdot N_{j}^{*}} \right\} \cdot {\sum\limits_{k \neq i}{{w_{i}(k)} \cdot {w_{j}^{*}(k)}}}}}}}} & {{Equation}\mspace{14mu} 11} \\{{R_{\hat{N}\hat{N}}\left( {{\mathbb{i}},j} \right)} = {{E\left\{ {{\hat{N}}_{i} \cdot {\hat{N}}_{j}^{*}} \right\}} = {{E\left\{ {\Delta\;{H_{i} \cdot \Delta}\; H_{j}^{*}} \right\}} + {{R_{N\; N}\left( {{\mathbb{i}},j} \right)} \cdot \left\lbrack {1 - w_{i} - w_{j}^{*} + {\sum\limits_{k}{{w_{i}(k)} \cdot {w_{j}^{*}(k)}}}} \right\rbrack}}}} & {{Equation}\mspace{14mu} 12}\end{matrix}$wherein i and j denote indices of the receive antennas and theexpectation operator E{ } is applied over a certain frequency range andover time. For example, i=0 and j=0 refers to noise variance of thefirst antenna.

The desired noise, assuming channel estimation errors and ignoring highorder noise terms, is given by:

$\begin{matrix}{{R_{NN}^{des}\left( {{\mathbb{i}},j} \right)}\overset{\sim}{=}{{R_{N\; N}\left( {{\mathbb{i}},j} \right)} + {E\left\{ {\left( {H_{i} - {\hat{H}}_{i}} \right) \cdot \left( {H_{j} - {\hat{H}}_{j}} \right)^{*}} \right\}}}} & {{Equation}\mspace{14mu} 13} \\{{R_{N\; N}^{des}\left( {{\mathbb{i}},j} \right)}\overset{\sim}{=}{{E\left\{ {\Delta\;{H_{i} \cdot \Delta}\; H_{j}^{*}} \right\}} + {{R_{NN}\left( {{\mathbb{i}},j} \right)} \cdot \left\lbrack {1 + {\sum\limits_{k}{{w_{i}(k)} \cdot {w_{j}^{*}(k)}}}} \right\rbrack}}} & {{Equation}\mspace{14mu} 14}\end{matrix}$

Assuming the channel estimation error is small relative to the noiseterms, the noise correction factor is given by:

$\begin{matrix}{{B\left( {{\mathbb{i}},j} \right)} = \frac{\left( {1 + {\sum\limits_{k}{{w_{i}(k)} \cdot {w_{j}^{*}(k)}}}} \right)}{\left( {1 - w_{i} - w_{j}^{*} + {\sum\limits_{k}{{w_{i}(k)} \cdot {w_{j}^{*}(k)}}}} \right)}} & {{Equation}\mspace{14mu} 15}\end{matrix}$

According to the previous example, the channel estimation weights are{¾, ¼} and the noise correction factor will be:

$\begin{matrix}{B = {\frac{\left( {1 + \left( \frac{1}{4} \right)^{2} + \left( \frac{3}{4} \right)^{2}} \right)}{\left( {1 - {2 \cdot \frac{3}{4}} + \left( \frac{1}{4} \right)^{2} + \left( \frac{3}{4} \right)^{2}} \right)} = {\frac{\frac{26}{16}}{\frac{26}{16} - \frac{24}{16}} = 13}}} & {{Equation}\mspace{14mu} 16}\end{matrix}$which is the desired correction factor for correcting the noiseestimation error.

In an embodiment, the bias correction factor can be similar for allfrequencies, and differ for different elements in the R_(NN) matrix. Inanother embodiment, the bias correction factor can be different for eachelement of the R_(NN) matrix and for each frequency range.

In an embodiment, noise estimation unit 44 calculates a noise estimateby processing the received reference symbols, noise correction unit 56calculates the noise correction factor of Equation 14, and then noiseestimation unit 44 multiplies the noise estimate by the noise correctionfactor, to produce a corrected noise estimate. In an embodiment, thecorrection factor is different for each element in the R_(NN) matrix.

FIG. 2 is a flow chart that schematically illustrates a method forsignal reception using noise estimate correction, in accordance with anembodiment of the present disclosure. The method begins at a receptionoperation 60, with RX FE 36 of UE 24 receiving a downlink signal fromBTS 28. The received downlink signal comprises data-carrying symbols andreference symbols. At a weighting operation 64, channel estimation unit40 applies weights to the received reference symbols, as explainedabove. At a channel estimation operation 68, channel estimation unit 40estimates the channel response based on the weighted reference symbols.

At a noise correction operation 72, noise correction unit 56 calculatesa noise correction factor based on the weights that are applied by unit40. At a noise estimation operation 76, noise estimation unit 44estimates the noise based on the estimated channel response and thenoise correction factor. At a decoding operation 80,equalizer/demodulator 48 uses the corrected noise estimate to demodulatethe data-carrying symbols of the downlink signal.

In the embodiments described above, channel estimation unit 40 assignsthe weights W according to channel estimation considerations,irrespective of noise estimation or noise correction. Then, noisecorrection unit 56 calculates the noise correction factors B based onthe given weights. In alternative embodiments, unit 40 chooses theweights while considering the noise correction factor that will becalculated by unit 56. In an example embodiment, a maximum value of thenoise correction factor is predefined. Unit 40 adjusts the weights sothat the resulting noise correction factor will not exceed the maximumallowed value. This technique enables the UE to trade channel estimationaccuracy for noise estimation accuracy.

In some embodiments, unit 40 holds multiple sets of weights, and selectsa given set for applying to a given received signal. In an exampleembodiment, each set of weights corresponds to a certain spectral range,and unit 40 selects the set that corresponds to the frequencies on whichthe signal is received. In some embodiments, unit 56 calculates a singlenoise correction factor for two or more of the sets of weights.

In some embodiments, noise correction unit 56 holds a list ofpre-calculated or predefined values of the noise correction factor B.Unit 56 selects the appropriate noise correction factor from the list byapplying a certain criterion to the channel estimation weights W. In anexample embodiment, unit 56 calculates the ratio between the weightassigned to a given reference symbol and the weights assigned to theother reference symbols, and chooses a value of B from the listaccording to this ratio. In alternative embodiments, unit 56 can use anyother suitable selection criterion.

Although the embodiments described herein mainly address downlinktransmissions and noise estimation in mobile terminals, the disclosedtechniques can also be used for noise estimation in uplink signals,e.g., in a BTS receiver. Although the embodiments described hereinmainly address LTE systems, the methods and systems described herein canalso be used in other applications, such as in WCDMA, WiMAX and LTE-Asystems.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present disclosure is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present disclosure includes both combinations andsub-combinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

The invention claimed is:
 1. A method, comprising: at a receiver,receiving a signal comprising reference symbols that is sent over acommunication channel from a transmitter to the receiver; estimating aresponse of the communication channel by applying one or more weightingvalues to a group of the reference symbols transmitted in respectivetime-frequency bins; computing an estimate of a noise level in thereceived signal based on the estimated response of the communicationchannel and on the same group of the reference symbols over which theresponse of the communication channel was estimated; computing, based onthe weighting values, a noise correction factor that compensates for adeviation in the estimate of the noise level caused by estimating boththe response of the communication channel and the noise level based onthe same group of the reference symbols that were transmitted in thesame respective time-frequency bins; correcting the estimate of thenoise level by the noise correction factor; and decoding the receivedsignal based on the corrected estimate of the noise level.
 2. The methodaccording to claim 1, comprising calculating channel quality feedbackfor the communication channel based on the estimated response of thecommunication channel and the corrected estimate of the noise level. 3.The method according to claim 1, wherein receiving the signal comprisesreceiving two or more versions of the signal via two or more antennas,respectively, wherein computing the estimate of the noise levelcomprises calculating multiple noise covariances based on the versionsof the signal, and wherein computing the noise correction factorcomprises calculating multiple noise correction factors to be appliedrespectively to the noise covariances.
 4. The method according to claim1, wherein applying the weighting values comprises selecting theweighting values from among multiple sets of the weighting values. 5.The method according to claim 4, wherein correcting the estimate of thenoise level comprises applying a single noise correction factor for atleast two of the sets of the weighting values.
 6. The method accordingto claim 1, comprising choosing one or more of the weighting values soas to improve the estimate of the noise level.
 7. The method accordingto claim 6, wherein choosing the weighting values comprises defining amaximum value for the noise correction factor, and adjusting theweighting values such that the noise correction factor does not exceedthe maximum value.
 8. The method according to claim 1, wherein computingthe noise correction factor comprises selecting the noise correctionfactor from a list of predefined noise correction factors by applying aselection criterion to the weighting values.
 9. The method according toclaim 1, wherein receiving the signal comprises receiving a Long TermEvolution (LTE) signal.
 10. Apparatus, comprising: a receiver front-end,which is configured to receive a signal comprising reference symbolsthat is sent from a transmitter to the receiver over a communicationchannel; a channel estimation unit, which is configured to estimate aresponse of the communication channel by applying one or more weightingvalues to a group of the reference symbols transmitted in respectivetime-frequency bins; a noise estimation unit, which is configured tocompute an estimate of a noise level in the received signal based on theestimated response of the communication channel and on the same group ofreference symbols over which the response of the communication channelwas estimated; and a correction unit, which is configured to compute,based on the weighting values, a noise correction factor thatcompensates for a deviation in the estimate of the noise level caused byestimating both the response of the communication channel and the noiselevel based on the same group of the reference symbols that weretransmitted in the same respective time-frequency bins, and to correctthe estimate of the noise level by the noise correction factor.
 11. Theapparatus according to claim 10, and comprising a channel qualityestimation unit, which is configured to calculate channel qualityfeedback for the communication channel based on the estimated responseof the communication channel and the corrected estimate of the noiselevel.
 12. The apparatus according to claim 10, wherein the receiverfront-end is configured to receive two or more versions of the signalvia two or more antennas, respectively, wherein the noise estimationunit is configured to calculate multiple noise covariances based on theversions of the signal, and wherein the noise correction unit isconfigured to calculate multiple noise correction factors to be appliedrespectively to the noise covariances.
 13. The apparatus according toclaim 10, wherein the channel estimation unit is configured to selectthe weighting values from among multiple sets of the weighting values.14. The apparatus according to claim 13, wherein the noise estimationunit is configured to apply a single noise correction factor for atleast two of the sets of the weighting values.
 15. The apparatusaccording to claim 10, wherein the channel estimation unit is configuredto choose one or more of the weighting values so as to improve theestimate of the noise level.
 16. The apparatus according to claim 15,wherein the channel estimation unit is configured to choose theweighting values by defining a maximum value for the noise correctionfactor, and adjusting the weighting values such that the noisecorrection factor does not exceed the maximum value.
 17. The apparatusaccording to claim 10, wherein the correction unit is configured toselect the noise correction factor from a list of predefined noisecorrection factors by applying a selection criterion to the weightingvalues.
 18. The apparatus according to claim 10, wherein the receiverfront-end is configured to receive the signal in accordance with a LongTerm Evolution (LTE) specification.
 19. A mobile communication terminalcomprising the communication apparatus of claim
 10. 20. A chipset forprocessing signals in a mobile communication terminal, comprising thecommunication apparatus of claim 10.